CVE-2025-69196

While testing the OAuth Proxy implementation, it was noticed that the server does not properly respect the resource parameter submitted by the client in the authorization and token request. Instead of issuing the token explicitly for this MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization.

Affected File:
https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828

Affected Code:

self._jwt_issuer: JWTIssuer = JWTIssuer(
    issuer=str(self.base_url),
    audience=f"{str(self.base_url).rstrip('/')}/mcp",
    signing_key=jwt_signing_key,
)

Since the issued access and refresh tokens do not include information about the resource the token was issued for, it is impossible for the MCP server to properly verify whether the token was issued for it, hence violating the requirement of doing so demanded by the specification. Being able to verify whether the token was issued for the target MCP server enforces the protections offered by the steps proposed by the specification and the Resource Indicators OAuth extension.

Therefore, this misconfiguration exposes all MCP server setups using the FastMCP OAuth Proxy to an attack where an adversary creates a malicious MCP server that advertises the benign OAuth Proxy authorization server as its own authorization server. Once a victim completes an OAuth flow with this malicious MCP server, authenticating against the AS, the adversary can extract the token received at the malicious MCP server and use it to access other MCP servers (the benign ones) that also use the same AS, including the tools and resources they expose.

Steps to reproduce:

1. Extract the provided PoC environment.
2. Enter the client_id and client_secret of a GitHub App you control into the mcp-server-proxy.py script.
3. Start the benign MCP server using an OAuth Proxy (in this case the GitHubProvider): python3 mcp-server-proxy.py.
4. Start the malicious AS: python3 mal_auth_server.py.
5. Start the malicious MCP server: python3 attacker_server.py.
6. Connect the client to the malicious MCP server: python3 client.py.
7. Complete the OAuth flow.
8. Observe in the logs of the malicious MCP server that the request to the benign MCP server with the stolen token returned a 200 status code.

Mitigation

To mitigate this vulnerability, it is recommended to issue tokens specifically for the MCP server submitted in the authorization URL’s resource GET parameter. In this way, the receiving MCP server will be able to properly verify that the token was indeed issued for it, allowing it to reject tokens stolen by an attack like the one demonstrated above.

This vulnerability allows an adversary to steal a victim’s authentication material for a benign MCP server using the FastMCP OAuth Proxy. The severity of this issue was decreased to Medium due to the consent screen showing the name of the MCP server the OAuth Proxy was intended for. However, a victim might not see it or get otherwise convinced by the attacker to ignore it, and overall this does not act as a proper mitigation for this issue.

The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.

Kodem intelligence

1. What is CVE-2025-69196? CVE-2025-69196 is a high-severity incorrect authorization vulnerability in fastmcp (pip), affecting versions < 2.14.2. It is fixed in 2.14.2. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
2. Which versions of fastmcp are affected by CVE-2025-69196? fastmcp (pip) versions < 2.14.2 is affected.
3. Is there a fix for CVE-2025-69196? Yes. CVE-2025-69196 is fixed in 2.14.2. Upgrade to this version or later.
4. Is CVE-2025-69196 exploitable, and should I be worried? Whether CVE-2025-69196 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2025-69196 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2025-69196? Upgrade fastmcp to 2.14.2 or later.